Vortex-frequency flowmeter

ABSTRACT

The present invention relates to a vortex frequency flow meter for determining the rate of flow of a liquid or gaseous medium through a pipeline with an obstructor (bluff body) mounted therein, said obstructor comprising lateral surfaces arranged essentially parallel to the flow and terminating in both directions of flow at vortex shedding edges, whereby at least one sensor for detecting vorteces periodically shedding from the shedding edges is disposed in at least one of said lateral surfaces. It is the object of the invention to develop such a vortex frequency flow meter in such a manner that it allows not only measuring operation in both directions of flow, but also a determination of the direction of flow using simple means. This task is solved in that the at least one sensor ( 16 ) is disposed off-centre—as seen in the direction of flow ( 20, 21 )—between the shedding edges ( 8, 8.1; 9, 9.1 ).

[0001] The present invention relates to a vortex frequency flow meterfor determining the rate of flow of a liquid or gaseous medium through apipeline in accordance with the preamble of claim 1.

[0002] Vortex frequency flow meters utilize the periodical vortexshedding at a blunt obstructor (bluff body) located in the fluid flow.Hereby, the phenomenon exists that vorteces are shedded alternatinglyfrom opposing sides of the obstructor surface facing the flow. Thereby,a so-called von-Karman vortex street is created, i.e. the vortecesremain active for a certain distance behind the obstructor in the flowprior to being desolved. Vortex frequency flow meters utilize thefinding that for certain obstructor profiles there exists a lineardependency between the frequency of vortex shedding and the speed offlow over a large area of low speeds, in other words, the speed of flowand therewith the flow volume of the fluid through the pipeline can bedirectly derived from determining this frequency. Thus, besides anobstructor, sensor means for determining the vortex sheddings or,respectively, the changes in parameters of the flowing fluid (e.g.pressure, speed, temperature) resulting thereform form part of ameasuring assembly of the type of a vortex frequency flow meter.

[0003] In the case of vortex frequency flow meters first described inliterature and firstly commercially utilized the obstructor consists ofa rod chaped profile extending diametrally in the cross section of theflow. Examples for vortex frequency flow meters with such obstructorbodies can be found in GB 1 401 272, U.S. Pat. No. 4,206,642, U.S. Pat.No. 4,285,247, DE 37 14 344 C2, DE 41 02 920 C2, U.S. Pat. No.3,979,954, EP 0 077 764, U.S. Pat. No. 4,434,668, U.S. Pat. No.4,922,759, U.S. Pat. No. 5,214,965, U.S. Pat. No. 5,321,990 and EP 0 666468.

[0004] Later vortex frequency flow meters with ring shaped obstructorswere developed. In comparison with rod shaped obstructors these exhibit,in the case of identical absolute blocking of the cross section of aflow (loss of pressure), a smaller width of profile. Resulting therefromis a higher vortex frequency at identical flow velocities, i.e. themeasuring accuracy is improved in comparison to rod shaped obstructors.Examples for vortex frequency flow meters utilizing ring shapedobstructors can be found in GG 1 502 260, WO 88/04410, DE 32 20 539, DE28 02 009, U.S. Pat. No. 5,170,671, U.S. Pat. No. 5,289,726, JP 560 22963, JP 59 19 8317, JP 11 48 912, JP 11 48 913 and JP 11 48 914. Vortexfrequency flow meters with ring shaped obstructors were never realizedin practice to-date, however.

[0005] DE 28 02 009 describes a vortex frequency flow meter with a ringshaped obstructor. In one embodiment this obstructor exhibits arectangular cross section, i.e. it has lateral surfaces lying inparallel to the direction of flow and terminating at vortex sheddingedges in both directions of flow, whereby the cross section of theobstructor between the shedding edges is symmetrical in the direction offlow as well as perpendicular to that. Thus, it is suitable formeasuring the flow in both directions. Radial tie bars serve to keep theobstructor mounted concentrically within the pipeline. For the purposeof determining the vortex shedding at the obstructor pressure orvelocity sensitive measuring sensors are provided at the obstructoritself or in its vicinity, for example, pipeline wall. More detailedspecifications regarding the location and construction of such measuringsensors have not been disclosed in DE 28 02 009.

[0006] GB 1 401 272 describes a vortex frequency flow sensor with a rodshaped obstructor likewise allowing for flow measurements in bothdirections. This obstructor is provided radially and axially centricallywith a through bore extending from one lateral surface of the obstructorto the other and being closed on both sides by membranes aligned in aformfit manner with the lateral surfaces. The through bore is filledwith oil, so that the membranes are hydraulically communicating. Withinthe through bore a piezoelectrical sensor is disposed which detects thepressure pulses transmitted through the membranes because of the vortexsheddings via the oil medium.

[0007] It is the object of the present invention to provide a vortexfrequency flow meter not only allowing measuring operation in bothdirections of flow, but also allowing determination of the direction offlow using simple means.

[0008] This task is solved in accordance with the invention by a vortexfrequency flow meter according to the features of claim 1.

[0009] Simply by virtue of the fact that the at least one sensor, whichis present and required anyway, is disposed in the direction of flowoff-centre (not in the middle) between the shedding edges, it is notonly possible to measure in both directions of flow, but in addition toalso determine the direction of flow. This effect results from the factthat the change in parameters, e.g. pressure, temperature and velocity,resulting from the vortex shedding are different in the two directionsof flow due to the axial displacement of the at least one sensor asopposed to a central disposition between the shedding edges, so thatbased upon these differences the direction of flow can be detected.Additional expenditure as regards the design of the vortex frequendyflow meter as well as additional measuring facilities ore not requiredfor this solution. However, this advantage comes at the expense of ahigher complexity of data processing in that the actual measuredfrequency and amplitudes is compared with frequency amplitude pairsdetermined under defined conditions whose combinations are uniquelyassociated with a certain direction of flow.

[0010] In a further embodiment of the invention the complexity of signalprocessing can be reduced in that the at least one sensor is associatedwith a second sensor disposed displaced in the direction of the flowcompared with the first and disposed on the same thread of flow. In suchan arrangement of sensors the direction of flow can be determinedwithout comparison with stored frequency amplitude pairs in that theactual measured amplitudes are directly compared with each other wherebythe direction of flow is determined from the amplitude difference.Possibly, unter suitable conditions, the temporal displacement of thesignals of both sensors can be used for determining the direction offlow.

[0011] Instead of the two sensors being displaced one behing the otherin the direction of flow, the sensors may be distributed over the heightof a rod shaped obstructor or over the circumference of a ring shapedobstructor respectively, and disposed in a manner displaced against eachother in the direction of flow. Such an arrangement would yield, inaddition to the advantage of the determination of the direction of flow,the additional advantage of allowing the detection of a symmetry offlow. It is apparent that using a higher number of sensors increases theaccuracy of the detection of a symmetry.

[0012] It is of advantage if the sensors are disposed in opposingdirections off-centre between the shedding edges, i.e. their spacingbeing as large as possible, because in that case the signal differencein both directions of flow is at its highest.

[0013] In view of this, it is advantageous to utilize microsensors inrealizing the invention. Such sensors can be disposed, due to theirsmall dimensions, in a very near vicinity of the shedding edges so thaton the one hand an optimum large spacing between axially ?despaced?sensors ensues and on the other hand a strong signal is generated.

[0014] A second advantage ensues when in embodying the invention withmeasuring points lying on opposite sides of the lateral surfaces of theobstructor these measuring points are connected via through bores. Thisserves to add the respective drops and boosts of pressure so thatcompaired to measuring points without such a connection a doublepressure amplitude ensues resulting in a very high measuringsensitivity. Thus, this solution would be most suitable for differentialpressure sensors.

[0015] Furthermore, it is of advantage if the through bores are closedby membranes essentially aligned in a formfit manner with the surfacesof the exterior side and the interior side. This serves to preventblocking of the through bores as well as the generation of perpendicularflow through the through bores disturbing the generation of vorteces.

[0016] The invention is subsequently further illustrated by means ofembodiment examples. The accompanying drawing shows in:

[0017]FIG. 1 a principal cross section of a vortex frequency flow metermounted in a pipeline with a ring shaped obstructor and a von-Karmanvortex street created behind the same;

[0018] FIGS. 2-8 a principal cross sectrion of a vortex frequency flowmeter mounted in a pipeline with a rod shaped obstructor and varioussensor arrangements;

[0019]FIG. 9 a principal pressure time diagram to illustrate the signaldifferences of both directions of flow;

[0020]FIG. 10 a section A-A according to FIG. 2 enlarged in scale;

[0021]FIG. 11 a section B-B according to FIG. 3 enlarged in scale;

[0022]FIGS. 12, 13 perspective views of vortex frequency flow meterswith a ring shaped obstructor and various sensor arrangements; and

[0023]FIG. 14 various possible cross sections of obstructors.

[0024]FIG. 1 shows a pipeline 1 in which a vortex frequency flow meter 2is mounted. Said flow meter consists of an exterior clamping ring 3 andan interior obstructor ring 4, said obstructor ring 4 being rigidlyconnected with the clamping ring 3 by means of three bars disposedradially in angular spacements of 120° (not shown in FIG. 1). Such bars5 are shown in FIGS. 12 and 13, whereby in these examples two or four,respectively, bars 5 provided for holding the obstructor ring 4.Clamping ring 3 serves for mounting the vortex frequency flow meter 2inside the pipeline 1 in that being clamped between two flanges notshown in detail here. Interior cross section corresponds to the interiorcross section R₀ of the pipeline 1, so that the interior wall 6 of thisis continued by the interior side of the clamping ring 3 and no vortecesensue this point. The surface 7 of the obstructor ring 4 facing the flow(the direction of flow is indicated by the arrow 20) is designed as aflowwise blunt surface oriented perpendicular in relation to the flowwhich is limited internally and on the outside by sharp shedding edges 8and 9. At these shedding edges 8, 9 ring vorteces 10 and 11 are sheddedalternatingly with identical frequency whereby the ring vorteces 10 withlarger diameters are associated with the shedding edge 8 and the ringvorteces 11 with smaller cross sections are associated with the sheddingedge 9. As the obstructor ring 4 is held by the bars concentricallyinside the pipeline 1 in case of a fully developed pipeflow, saidobstructor ring lies on a circular curve of equal velocity as can beseen from the turbulent velocity profile 12 shown in FIG. 1. Thus, thevortex shedding can happen in a very homogeneous manner, so that thering vorteces 10 and 11, respectively, remain active for a relativ longperiod behind the vortex frequency flow meter 2 as a so-calledvon-Karman vortex street before they become dissolved.

[0025] With the rod and ring shaped obstructors 4 chozen for the exampleembodiments the cross sections 13 thereof are rectangular. Such a crosssection 13 is symmetrical in relation to its two main axes 19, 22 in thedirection of flow and perpenducular thereto 20, 21, so that identicalparameters ensue when the vortex frequency flow meter 2 is hit withfluid against the direction of flow 21 (FIG. 1) (the vorteces 10, 11then detach themselves from the shedding edges 8.1 and 9.1). In otherwords, with such a cross section a certain flow velocity leads to anidentical frequency for both directions of flow. Despite the option ofmeasuring the volume flow in both directions the complexity of signalprocessing thus remains low.

[0026] In FIG. 14, by way of example, further cross sections 13 of theobstructors 4 are shown allowing measurements to be taken in bothdirections of flow. In the illustrations below we continue to use arectangular cross section by way of example (FIGS. 14.2, 14.3), while itis noted, however, that what is said is equally aplicable to other crosssections.

[0027] The sensors built into the obstructor ring 4 are microsensors 16(FIGS. 10, 11). They are merely indicated symbolically by circles inFIGS. 2 through 8 and 12 and 13. As explained above, the vortecesshedded at the shedding edges 8, 9 or 8.1, 9.1, respectively, lead tolocal variations in velocity and pressure. Thus, all measuringprinciples are suitable which allow a detection of these values or ofparameters dependent upon these values. Examples for suitable sensorswould thus include: differential pressure sensors, absolute pressuresensors, total flow resistance sensors, flow friction sensors, heatdissipation sensors and heat distribution sensors. Such sensor types aregenerally known to the expert in the art, so that these constructionprinciples can be converted into microtechnology, i.e. these known typesof sensors can be miniturized. The microsensors 16 are thus shown inblack box type in FIG. 10, 11 as only the measuring principle realizedby means of the microsensors is of relevance, not the exact designthereof.

[0028] In the chosen embodiment examples differential micropressuresensors 16 are being utilized while the invention is by no means limitedto such sensors. In measuring by means of differential pressuremicrosensors 16 two measuring points 16.1 and 16.2 are provided whichlie at the lateral surfaces 17, 18 of the cross section 13. Themeasuring points 16.1, 16.2 are interconnected by a through bore 23. Themeasuring points 16.1, 16.2 may either be differential pressuremicrosensors 16 or the outlets of the through bores 23 as shown in FIGS.10 and 11. Through the through bores 23 the pressure differences on thelateral surfaces 17, 18 are superimposed. As due to the alternatingshedding of vorteces increase of pressure on the one side 17, 18 meetswith a drop in pressure of approximately equal scale on the other side18, 17 a doubling of signal amplitude ensues, i.e. the measuring signalsamplified largely materially increasing the measuring sensitivity.

[0029] The arrangement of sensors for detecting the direction of flow inaccordance with the invention is subsequently further illustrated bymeans of FIGS. 2 through 8. These figures show a vortex frequency flowmeter 2 with a rod shaped obstructor 4 built into a pipeline 1. Aturbulent flow profile 12 is present.

[0030] In principle, one sensor 16 is sufficient for determining theflow volume and the direction of flow. This most simple case isrepresented in FIG. 2. As can be seen from this drawing, thedifferential pressure microsensor 16 is disposed at the level of thepipe axis and displaced against the direction of flow 20, i.e. towardsthe shedding edges 8, 0. With this arrangement the pressure amplitudemeasured, based upon identical flow velocities, is larger when the flowcomes from the direction 20 than if it comes from the direction 21. Thisis shown in the diagram according to FIG. 9, in which curve A isassociated with the direction of flow 20 and curve B with the directionof flow 21. Whether, since one amplitude value can be associated withtwo frequencies, i.e. two velocities, that is in the direction of flow20 or 21, this amplitude on its own is not sufficient to determine thedirection of flow 20, 21. What must be utilized in addition is a fieldof characteristic and data of frequency amplitude pairs determined underdefined conditions with which the actual measures values can becompared. Thus, in an arrangement with only one displaced sensor 16,there is an increased complexity of date processing.

[0031] This complexity is avoided using an arrangement according to FIG.3. Hereby, two differential pressure microsensors 16 are provided whichare disposed at the same level, in this case at the level of the pipeaxis. Both differential pressure microsensors 16 are similarly displacedas against the main axis 19 of the cross section 13 towards the sheddingedges 8, 8.1 and 9, 9.1, respectively (FIG. 11). This displacement ofsensors 16 against the two directions of flow 20, 21 increases thecertainty of determination of direction and predominantly reduces thecomplexity of date processing, because a comparison of signal amplitudescan be executed directly (FIG. 9) without having to revert to a storedfield of characteristic data as in the arrangement according to FIG. 2.A further advantage as opposed to that arrangement lies in an increasedredundancy of measurements.

[0032] Moreover, an increased redundancy of measurements as opposed tothe arrangement according to FIG. 2 is obtained by a sensor distributionaccording to FIG. 4. Hereby, two differential pressure microsensors 16are displaced in the same direction, i.e. towards the shedding edges 8,9 by equal distances, whereby the distance r_(M) of the sensors 16 inrelation to the pipe axes is identical. Besides a determination of theflow volume and the direction of flow 20, 21, this arrangement allowsthe detection of flow asymmetries if a respective complexity of dateprocessing is utilized. As regards the detection of the direction offlow 20, 21, there is the same disadvantage as with the sensorarrangement according to FIG. 2. However, this disadvantage can beovercome, in a manner analogue to the solution according to FIG. 3, bydoubling the sensors 16, als shown in FIG. 5.

[0033] However, this disadvantage is avoidable by means of a more simplesolution shown in FIG. 6. Hereby, two differential pressure microsensors16 disposed at equal distances r_(M) from the pipe axes are disposeddisplaced in opposing directions. As these sensors, in a case ofsymmetrical flow, detect equal local flow velocities, i.e. frequencies,owing to the equality of distance in relation to the pipe axis, thisarrangement corresponds to the sensor arrangement according to FIG. 3,as regards the detection of the direction of flow 20, 21, while offeringthe advantage of allowing the detection of flow asymmetries.

[0034] It is apparent that increasing the number of sensors 16distributed over the height of the obstructor 4 increases the accuracyof detection of asymmetries. FIG. 7 shows a sensor arrangement in whicha third, central measuring point is added to the arrangement shown inFIG. 6. As there exists no comparison position for this central positionit is disposed centrically between the shedding edges 8, 9 and 8.1, 9.1.It would also be conceivable to displace this central measuering pointin the one or the other direction of flow 20, 21 in order to maximizethe signal amplitude of this measuring point for a certain maindirection of flow 20, 21.

[0035]FIG. 8 shows an obstructor 4 with four differential pressuremicrosensors 16 displaced in pairs, whereby the differential pressuremicrosensors 16 of one pair always exhibit equal distances r_(M1) orr_(M2) from the pipe axis. Similarly to the arrangement according toFIG. 5, this arrangement leads to a redundancy as regards the detectionof the direction of flow 20, 21.

[0036] As a further variation of a displacement of measuring points itwould be conceivable to displace all measuring points below the pipeaxis into one direction and all measuring points above the pipe axisinto the other direction.

[0037]FIGS. 12 and 13 show vortex frequency flow meters 2 with ringshaped obstructors 4 whose sensor arrangement is similar to that ofFIGS. 6 and 8, respectively. Thus, what was said in relation to rodshapes obstructors 4 applies also for these cases.

1. A vortex frequendy flow meter for detecting the flow volume of aliquid or gaseous medium through a pipeline (1) with an obstructor(bluff body) (4) mounted therein, said obstructor comprising lateralsurfaces (17, 19) arranged essentially parallel to the flow andterminating in both directions of flow (20, 21) at vortex shedding edges(8, 8.1; 9, 9.1), whereby the cross section of the obstructor (4)between the shedding edges (8, 8.1; 9, 9.1) in the direction of flow(20, 21) and perpenducular thereto is essentially symmetric and in atleast one of the lateral surfaces (17, 18) at least one sensor (16) fordetecting vorteces (10, 11) periodically shedding from the sheddingedges (8, 8.1; 9, 9.1), characterised in that the at least one sensor(16) for detecting the direction of flow (20, 21) is disposedoff-centre—as seen in the direction of flow (20, 21)—between theshedding edges (8, 8.1; 9, 9.1).
 2. Vortex frequency flow meteraccording to claim 1, characterised in that the obstructor (4) is rodshaped, ring shaped or at least partially ring shaped.
 3. Vortexfrequency flow meter according to claim 1 or 2, characterised in thatthe at least one sensor (16) is associated with a second sensor (16)disposed in a manner displaced in relation to the first one in thedirection of flow (20, 21) and lying in the same thread of flow. 4.Vortex frequency flow meter according to claim 2 or 3, characterised inthat at least two sensors (16) are provided which are distributed acrossthe height of the rod shaped or upon the circumference, respectively, ofthe at least partially ring shaped obstructor (4) and are displacedagainst each other in the direction of flow (20, 21).
 5. Vortexfrequency flow meter according to claim 4, characterised in that saidsensors (16) are displaced in pairs and the distance of the sensors (16)of one pair from the pipe axis is identical.
 6. Vortex frequency flowmeter according to one of the claims 3 through 5, characterised in thatsaid sensors (16) are disposed off-centre in opposing directions betweenthe shedding edges (8, 8.1; 9, 9.1).
 7. Vortex frequency flow meteraccording to one of the preceding claims, characterised in thatmeasuring points (16.1, 16.2) disposed on opposite sides of the lateralsurfaces (17, 18) of the obstructor (4) are interconnected by a throughbore (23).
 8. Vortex frequency flow meter according to claim 7,characterised in that said through bores (23) are closed on both sidesby membranes aligned in an essentially formfit manner with the surfaceof the lateral surfaces (17, 18).
 9. Vortex frequency flow meteraccording to one of the preceding claims, characterised in that saidsensors (16) are microsensors.